Genetically Engineering Cotton for Insect Resistance
The DNA code mostly contains instructions for protein synthesis. The code is read in groups of three nucleotides and each triplet of nucleotides codes for one of the twenty amino acids which link together in a polypeptide chain to form a protein. The code is universal, so the same code applies in nearly all living organisms. Some triplets have special functions and direct protein synthesis to start or stop. Protein synthesis occurs in ribosomes where a copy of the gene coding for a protein (mRNA) is translated to produce a protein. Some proteins may be consist of several polypeptide chains and the genes required to do this are collectively called a transcription unit.
Fig. 2 Diagram showing how genes code for proteins
Bacterium also contain small circular loops of DNA called plasmids which are not essential to the bacterium but can be useful in certain environmental conditions such as resistance to antibiotics. Because bacterium are prokaryotic and don't have a nucleus plasmids are easy to obtain in pure form and can be introduced into other cells. Plasmids are also capable of independent self-replication, which makes them useful in multiplying useful DNA.
Bacteria also produce restriction enzymes, which can cut DNA at specific base sequences. Different restriction enzymes cut different base sequences and some make staggered cuts which leaves unpaired DNA ("sticky ends") and other cut leaving no unpaired DNA ("blunt ends").
Techniques used in genetically engineering cotton for insect resistance
The first step in inserting the Bt gene into the cotton plant is determining the Bt protein's amino acid sequence. Using the principles of the genetic code it is possible to construct a complementary DNA sequence called and oligonucleotide using an automated DNA synthesiser.
This oligonucleotide can then be used as a DNA probe to isolate the DNA from the Bascillus thuringiensis. It is made radioactive and when inserted into the bacteria it hybridises (attaches to the complementary base pairing) with the DNA sequence that codes for the Bt protein. The DNA binding to the probe becomes radioactive so it can be detected by x-ray film.
Fig. 3 DNA probe production
The gene is then isolated from the bacterium by using restriction enzymes and multiplyed in the bacterium E. coli through gene cloning. The gene is first inserted into a plasmid from E. coli containing a gene coding for resistance to the antibiotics kanamycin and neomycin. The plasmid is cut with the same restriction enzyme as used to cut the Bascillus thuringiensis' DNA. The restriction enzyme cuts both the DNA and the plasmid leaving sticky ends on the resulting fragments that enable the Bt gene to be incorporated into the plasmid. The complementary ends pair and the enzyme DNA ligase is used to join them together.
Fig. 4 Bt gene insertion into E. coli plasmid
The plasmid is then introduced into the E....